Why Is Most DLC Sub-Par?

Hardcore Gamer: With the amount of development focus being placed on DLC, even before the game is finished in some cases, you'd think it would generally be as good as the main game, but that usually isn't the case. So, why exactly is downloadable content of the same quality as the core game such a rarity?

recently ive bought LBPvita Time Saver bundle, some costumes, then some free ones. Left Behind DLC, some pinball tables and then got free KZ:M maps. b4 that i havent bought DLC since socom confrontations and warhawk tbh. im not that crazy about DLC. if i want it, its good and/or there is a good reason for it.

And yet still, there's a lot of good to appreciate. Left Behind was a good DLC and big standalone adventures like Undead Nightmare are nice. And expansions like Dragonborn add a lot to the experince.

My favorite DLC of all time has to be the Minerva's Den DLC for Bioshock 2, which I honestly beleive is the overall pinnacle of the Bioshock series as a standalone adventure. A 5-hour, $10 DLC with excellent production and unique assets, challenges, and additions over the base game.

To me, I think DLC should be more like "expansion packs" of the late 90's to early 2000's (think: Medal of Honor Allied Assault: Spearhead/Breakthrough, or Command and Conquer Red Alert 2: Yuri's Revenge).

Additional weapons, skins, and really really minor things should just be called "add-ons".